Before, the mheap structure was in the bss,
but it's quite large (today, 256 MB, much of
which is never actually paged in), and it makes
Go binaries run afoul of exec-time bss size
limits on some BSD systems.

Fixes #4447.

R=golang-dev, dave, minux.ma, remyoudompheng, iant
CC=golang-dev
https://golang.org/cl/7307122